while you tagged jquery .. this is a jquery code .. but be sure to include jquery first

var parText = $('#wrapper p').text(); // get text of p
var parSplit = parText.split("\n"); // split text by \n
var lastLine = parSplit[parSplit.length-1]; // get last line
parText.replace(lastLine , ''); // replace last line with nothing
$('#wrapper p').append('<span>'+lastLine+'</span>'); // append the last line with `<span>lastline</span>` and give it a style you need

DEMO

another DEMO with slideDown effect